The Boston Teachers Union is poised to endorse Councilors Rob Consalvo and Felix G. Arroyo for mayor, potentially bolstering their campaigns with thousands of foot soldiers in the crucial days before Tuesday’s preliminary election.

The dual endorsements are expected to be finalized Wednesday afternoon during a general membership meeting that will probably draw hundreds of teachers who are eager to influence the direction of the city’s school system.

Education has been a significant issue in the race and a fracture line has emerged over charter schools, which rarely employ unionized teachers. Many teachers oppose election of a mayor who calls for more charter schools.

Both Consalvo and Arroyo are against allowing more charter schools to open and have been vocal in their support for teachers.

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“We feel there are a number of candidates who believe strongly in public education, but we think these two far and away separated themselves by being more thoughtful and for having a better grasp of what the schools need to move forward,” Richard Stutman, the union’s president, said in an interview.

The decision to back two candidates instead of one in a race in which voters can cast a single ballot is unusual for an organization making an endorsement, causing it to split resources and attention between two campaigns.

But equally unusual is the teachers union’s decision to endorse a mayoral candidate. The union last endorsed a mayoral candidate in 1991, when the group’s former president, Edward Doherty, unsuccessfully challenged Mayor Raymond L. Flynn.

Stutman likened the twin endorsements to those made in the past week by the editorial boards of The Boston Globe and The Boston Herald, which each recommended two candidates.

“Either one would be an excellent mayor,” Stutman said of Arroyo and Consalvo.

In some ways, the two endorsements reflect splintered support for the 12 mayoral candidates across the city, where polls have shown the race is too close to call.

A recent Globe poll found John Connolly, another councilor running for mayor, leading the pack, but he was favored by just 13 percent of those polled and many other candidates were clustered tightly behind him, meaning that any of them could prevail.

Whether the Teachers Union’s endorsements will elevate either Consalvo or Arroyo to the top two spots remains unclear. Some other candidates have more extensive field operations and money that could put them further ahead. The union hopes to offset that by enlisting their members to canvass neighborhoods on behalf of Arroyo and Consalvo, and the union also intends to publish fliers and run online advertisements.

The union says it has more than 5,000 members, including retirees, living in the city.

“Obviously when you have a group that large, it will have an impact,” said Larry DiCara, who unsuccessfully sought the union’s endorsement when he ran for mayor 30 years ago.

He said it could be enough to nudge one of them into the final two, especially as many voters are still weighing options and shifting support.

But he added the Teachers Union has less of an impact than a generation ago because fewer members live in the city.

In an interview, Consalvo said he would love to have the union’s endorsement.

“I think every little bit [of support] helps,” Consalvo said. “I think the election will be decided closely within a few hundred votes.”

He also stressed his support of teachers and expressed disappointment about how teachers and the schools have been demonized in the debate on overhauling public education.

“I will be the mayor who will work with the Teachers Union to move the Boston public schools forward,” Consalvo said.

In a statement, Arroyo expressed gratitude for the initial union nod, as well.

“As the proud son and husband of public school teachers, I am excited to have earned the support of the men and women who educate our children,” said Arroyo. “I strongly believe in supporting our public schools and working with our teachers to improve our education system, and I will gladly defend that work in the general election and as mayor.”

Initially, the Teachers Union was not planning to endorse a candidate before the preliminary because the membership was so divided over the candidates and was going to wait until the general election.

But Stutman said momentum for Arroyo and Consalvo picked up considerably after the union sponsored a mayoral forum last Wednesday, where the two candidates won over many teachers. Stutman said Arroyo’s passion about lifting all children out of poverty resonated with many members, while Consalvo’s zeal for policy and practical ideas appealed to others.

In backing the mayoral hopefuls, the union has to bend some of its own rules. Stutman said last week that an endorsement would require two-thirds support from the union’s executive committee, its political advisory committee, and its membership.

But Tuesday, he said the impending endorsements are technically just recommendations of who to vote for, enabling the motion in favor of Arroyo and Consalvo to pass with a simple majority.

The razor-thin poll results, Stutman said, also confirmed for union leaders that expediting an endorsement might influence the election, helping them to get a like-minded candidate in the final race.

The union has made clear its disappointment with candidates who support more charter schools, Connolly in particular. In a flier advertising the recent forum, the union chided Connolly for accepting an endorsement from Stand for Children, a national education organization that often is at odds with teacher unions.

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